Electrical testing in manufacturing plants: Beyond the bridge

—Sponsored article by Megger

In the past, measuring the capacitance and dissipation factor of a test specimen at manufacturing facilities was carried out using high-voltage sources and Schering bridges.

Even though Schering bridges are capable of producing good results in many applications, they’re time consuming and not always user friendly. Today, automatic-balancing bridges provide an innovative solution with very high accuracy and an easy interface.

The Megger CDAX 605 high-precision capacitance and dissipation factor test set is the most suitable option for high voltage C & DF measurement in factories. With the CDAX 605, working voltage is limited only by the characteristics of the reference capacitor or resistor.

This novel test set produces accurate results at frequencies up to 400 Hz, and is practical for use in a laboratory, as part of a factory production test facility, or in the field.

As with a conventional bridge, the CDAX 605 compares the test specimen against a reference capacitor or resistor but unlike a bridge, it doesn’t require manual tuning. Instead, it uses the automatic current comparative method to measure capacitance and dissipation factor.

Sophisticated self-calibration procedures using an internal ratio-arm bridge allow very high levels of accuracy to be achieved. For example, dissipation factor results are within ± 0.05% + 0.002% for measurements made at 50 or 60 Hz, with measurement currents between 15 μA and 300 mA. Absolute voltages and currents are measured with an accuracy of ± 0.1%.

A major benefit of the CDAX 605 is it can compare currents irrespective of their phase difference. Because of this unique feature, capacitive measurements can, in practice, be made using a reference resistor.

This is particularly useful when measuring the ratio of coupling capacitor voltage transformers (CCVTs), since a simple and relatively inexpensive resistive divider is all that’s needed as the reference element, rather than a costly gas-insulated reference capacitor.

Another unique characteristic of the CDAX 605 is it has one input channel for the reference capacitor or resistor, but two input channels for the specimens under test. Having two independent input channels allows for two test specimens to be connected and tested at the same time, resulting in time savings on the test—particularly useful in production line test applications where high throughput is important.

The automatic operation of the instrument, which has no front panel balancing knobs, is another invaluable time-saving feature. The CDAX 605 makes no sacrifices in the area of versatility. It can accommodate all standard test configurations, including UST, GST-GND and GST-guard, and it offers a wide measurement range, typically covering capacitance from 0 to 100 μFand dissipation factor from 0 to 10, as well as phase angle measurement from -180º to +180º.

The instrument is used in conjunction with an external PC and it is supplied with purpose-developed software that provides a convenient and notably intuitive user interface, as well as comprehensive data logging capabilities.

Support is also included for a LabView interface that further enhances the versatility of the instrument. It is likely that most purchasers will already have a suitable voltage source, together with reference capacitors and resistors. This means that they have all the additional equipment needed to form a complete dissipation factor and capacitance measuring system, with minimum expenditure.

The benefits of Megger’s innovative CDAX 605 high-precision capacitance and dissipation factor test set have already proved convincing for many leading organizations in the electrical power sector.

GE-Alstom, for example, has the instruments in its HV test laboratories, as has Trench Canada, a major supplier of CCVT equipment.

With the new CDAX 605, HV measurements of capacitance and dissipation factor can now be performed quickly and conveniently. The CDAX 605 delivers accurate results more efficiently and reliably than older instruments – or, to put it another way, this new test set undoubtedly goes well beyond the bridge!

Diego Robalino is a Senior Applications Engineer with Megger.

This article is an edited version of the original, which appears in the May 2016 edition of Megger’s Electrical Tester magazine.